It would be something like this:
exten => _7619998887777,1,AGI(
agi://127.0.0.1:4577/call_log)
exten => _7619998887777,n,Dial(SIP/${EXTEN:3,10}@voip-clientscompany.com,,tToR)
exten => _7619998887777,n,Hangup()
exten => _76.,1,AGI(
agi://127.0.0.1:4577/call_log)
exten => _76.,n,Dial(SIP/${EXTEN:3,10}@voip-normalcarrier.com,,tToR)
exten => _76.,n,Hangup()
That way, only the 9998887777 number will go to the client carrier, all other numbers use your normal carrier, and this is all using the same dial prefix.